1. Field of the Invention
The present invention is in the field of applications for and treatments of alloyed steel in the manufacture of instruments. It concerns an elastic component for a precision instrument, including the manufacture and use of the component.
2. State of the Art
In the manufacture of instruments, certain component elements are subject to exacting requirements with regard to their mechanical properties. Typical representative examples of such elements are the flexible guides, couplings and pivots that are used in the load cells of high-precision weighing instruments; or force/displacement transducers with their associated sensors. In force/displacement transducers, the linearity of the force/displacement relationship and the reproducibility of the mechanical properties are primary objectives. Common to all elements covered under this invention is the requirement that anelasticity, creep and mechanical hysteresis be minimized. In addition, the elements are to be corrosion-resistant and preferably non-magnetic.
Components of the aforementioned kind that are used in high-performance precision instruments, e.g., for high-performance load cells of precise balances, are made primarily of martensitic precipitation-hardened stainless steels. Normally, they are solution heat treated and aged at temperatures between 450xc2x0 C. and 550xc2x0 C. Commercially available types are known, e.g., under the designations 17-7PH, 17-4PH, or 13-8Mo. They are virtually free of creep but suffer from a large hysteresis: When a load is applied and then removed in short succession, there is practically no remanent deformation, but in the load/displacement diagram, the curves for increasing and decreasing loads do not coincide. With variations between batches and depending on the heat treatment, the temperature, and the in-use loads that they are subjected to, components made from this material exhibit a mechanical hysteresis of 1xc2x710xe2x88x924 to 5xc2x710xe2x88x924. Although this amount of hysteresis may be reduced and/or partially compensated by appropriate measures, it can never be made to disappear entirely and remains as an undesirable effect. The current state of the art offers numerous solutions to overcome this unwanted effect. The range of solutions encompasses the purely mathematical error compensation (U.S. Pat. No. 4,691,290), the compensation through the design of the fastening connection (DE-U-29612167) or through the sensor, and reaches as far as special alloys and methods for influencing the lattice structure and cell geometry to prevent Bloch wall (domain wall) friction (JP-A-59126760, DE-A-4034629). These measures are either expensive or not sufficiently reproducible, or they have a negative effect on certain elastic properties. Another possibility for achieving freedom from hysteresis lies in the application of austenitic steels. However, the available austenitic steels are optimized to be resistant to corrosion and have inadequate elastic properties; after cold work hardening they are suitable for springs, but not for transducers, due to their high proportion of creep. Newer austenitic steels are designed for higher yield stresses at high temperatures, but their creep properties, likewise, are unsatisfactory. Another problem arises from the magnetic properties of the steels. Particularly in balances, but also in other precision instruments where forces or force-related displacements are to be measured, the measurement cannot be allowed to be affected by the possible presence of a magnetic field. Therefore, as an additional requirement, the constructive elements of a precision instrument must be non-magnetic.
Thus, an object is to provide a corrosion-resistant, non-magnetic device whose deformation under an applied load is in linear proportion to the load, close to totally reversible, and free from hysteresis. The present invention solves this problem by providing an elastic component for a measuring device in which the component is subjected to mechanical stress. In exemplary embodiments, the inventive component
a) comprises an austenitic metal alloy that includes interstitial atoms, contains in proportion to its total weight less than two percent ferrite, less than two percent martensite, and more than eleven percent chromium, and has a crystalline texture with a nano-structure with blocked dislocations.
b) is made by a process that uses the alloy after it has been solution heat-treated and quenched, comprising process steps of
shaping the component from the metal alloy, which involves application of mechanical stress that causes cold-hardening at least in localized portions, and
thermal hardening in a temperature range from 200xc2x0 C. to 700xc2x0 C.
The component is of importance for applications in precision measuring instruments such as balances of high resolution. The component itself may contribute to the process of generating the measurement value as is the case in transducers, sensors or the like, where the relationship between stress and strain or between load and displacement is being used, or it may be employed in pivots, guides, couplings or the like.
Using components of this kind further presents the solution for producing
a) transducers incorporating an elastic component for measuring mass, weight, force, torque, angle, or displacement,
b) motion-guiding mechanisms for a precision measuring instrument incorporating an elastic component,
c) coupling elements used in a precision instrument in the form of an elastic component with a portion of locally reduced thickness defining the line of force introduction,
d) pivot elements used in a precision instrument in the form of an elastic component with an area of locally reduced thickness defining the pivoting axis.
The elastic component of the invention is based on a metal alloy whose structure is essentially austenitic. Alloy steel is preferred. In exemplary embodiments, the proportion of ferrite and/or martensite is limited to less than two percent, which ensures that the steel is non-magnetic, and a chromium content of more than 11 percent by weight provides the required resistance to corrosion. The crystalline texture has a nano-structure that anchors dislocations, resulting in low amounts of hysteresis, anelasticity and creep.
The special crystalline texture results from a hardening process that comprises cold hardening during the shaping of the component and aging (thermal hardening, precipitation hardening) after the component has been shaped.
The base material in the manufacture of the elastic component according to the inventive process is a stainless, austenitic metal alloy containing interstitial atoms, which has been solution heat-treated and quenched. Exemplary candidates for selection are iron or nickel alloys with a sufficiently high chromium content, and can be supplemented with other elements to increase corrosion resistance, and with a sufficiently high content of nitrogen or other suitable elements to supply the interstitial atoms. From the base material, a component is made by conventional metalworking techniques. The shaping of the component can take place in a single step or in several steps. The manufacturing process of the component comprises cold working or a mechanical surface treatment or a combination of both, at least in selected portions. This treatment increases the dislocation density in the treated areas and thus hardens the material. A subsequent so-called aging, which from here on will be referred to as thermal hardening, of the entire component at a temperature range from 200xc2x0 C. to 700xc2x0 C. serves to anchor the dislocations, which has a positive effect on the elastic properties.
Particularly advantageous properties are obtained locally if the shaping process results in a plastic deformation of more than ten percent to a depth of at least 50 xcexcm. Thus, component portions of locally reduced thickness as they occur in coupling and pivot elements will attain outstanding elasticity after the subsequent thermal hardening.
Thermal hardening has a particularly beneficial effect on the elastic properties if applied for more than five hours at temperatures below 480xc2x0 C., yet without extending the duration of this process step to the point where corrosion resistance is unacceptably lowered.
Further advantages result from the special embodiments of the component as will be discussed below.
The elastic components of this invention, made in accordance with the inventive process, are particularly suitable for applications in precision instruments. Thus, they can be employed in transducers that rely on the highest possible degree of linearity in the relationship between force and displacement or between stress and deformation to produce a measuring result. Or they can be applied in a motion-guiding mechanism that restrains certain degrees of freedom of movement, while leaving others totally without restraint. Or they are utilized in a precision instrument as elements with locally reduced thickness in the form of a coupling for defining the line of force introduction or in the form of a pivot element for defining the center of pivoting action. In a high-precision balance, all of these applications can occur either individually or in combination with each other.